= L-homocysteine
{c}
\b[L-homocysteine] is the L-configured form of <homocysteine>, a sulfur-containing <amino acid>. ChEBI specifies its systematic name as (2S)-2-amino-4-sulfanylbutanoic acid. The neutral representation has <molecular formula> $\mathrm{C_4H_9NO_2S}$, but the formula alone does not specify the <stereochemistry>.<Homocysteine in ChEBI>[CHEBI:17588, Definition, IUPAC Name, Formula and structure; compare generic CHEBI:17230.]{ref}
Its <stereocenter> has four different attachments: hydrogen, an amino group, a carboxyl group and the side chain $\mathrm{-CH_2CH_2SH}$. A mirror-image arrangement can retain the same atom inventory and connectivity while differing in configuration. This makes <L-homocysteine> a concrete example of why a <molecular formula> cannot distinguish <enantiomers>.
The stereochemical qualifier also matters in biochemical reaction notation. The accepted <adenosylhomocysteinase> reaction names <L-homocysteine> as one product of hydrolyzing <S-adenosylhomocysteine>. That explicit identity is more precise than reporting only a sulfur-containing product. Protonation and configuration remain separate descriptors: a record of a conjugate acid or base is not, merely for that reason, a different <enantiomer>.<Enzyme Nomenclature by the IUBMB>[EC3.13.2.1, accepted reaction.]{ref}
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